1. Field of the Invention
The present invention relates to an abrasive which can be advantageously used for dressing diamond segments of a core bit when boring a working material such as asphalt or concrete by the core bit, and a boring method using the abrasive.
2. Description of the Related Art
Conventionally, for boring a working material such as asphalt, concrete, or the like, a boring method is known in which a circular groove is cut into the working material wherein a core bit, which has diamond segments comprising diamond abrasive grains or the like attached to the distal end opening of a cylindrical core, is pressed against the working material while being rotated by a drive source such as a motor.
The diamond segments are formed by dispersing diamond abrasive grains or the like in a binder phase made by sintering a binder material such as a metal bond. To maintain an advantageous boring condition, the new diamond abrasive grains in the inner layers must appear in sequence after an appropriate amount of wear of the binder material, promoting what is termed a self-sharpening function or a dressing function, after diamond abrasive grains on the outermost surface have lost the capacity to function as a boring blade because of deterioration and defects.
FIG. 4 shows one example of a core bit having this type of structure, and is an exploded perspective drawing showing the distal end part. In this drawing, reference numeral 110 is a shank and 310 are diamond segments. As shown in FIG. 4, at the distal end part 110a, which is the open end of the cylindrical shank 110 that rotates around an axis, a plurality of diamond segments 310 are affixed at a predetermined distances in the circumferential direction so as to project from the distal end of the shank 110, for example, by soldering or laser welding. Furthermore, in the radial direction of the shank 110, the thickness of the diamond segments 310 is formed so as to be thicker than the thickness of the shank 110, that is, the distance between the inner circumferential surface and the outer circumferential surface of the shank 110. Moreover, the diamond segments 310 are formed by the diamond abrasive grains W or the like being dispersed in a binder phase made by sintering a binder material such as a metal bond.
However, in the case that the working material is hard, because the chips are reduced to fine particles, not only is realizing the dressing function of the diamond segments impossible, but the chips adhere to the binder material, the diamond abrasive grains are surrounded, and a ceramic-like state is formed having a surface that is dense and smooth. The promotion of friction heat is brought about due to what is termed loading. Thereby, as disclosed, for example, in Japanese Unexamined Patent Application, First Publication, No. Hei 4-319408, a boring method is used in which, while boring the working material, abrasive grains, such as alumina, silicon carbide, or molding sand and grinder powder, are supplied to the boring member as an abrasive that dresses the diamond segments. At the same time, the friction of the diamond segments due to the boring heat is reduced by cooling the diamond segments using a dry method in which a gas such as air is supplied to the boring member as a cooling fluid, or using a wet method in which a fluid such as water is supplied to the boring member.
Except for boring comparatively shallow holes, actual boring operations generally use a wet method that employs water or the like. In particular, in cases where a hole having a depth of one meter or greater is necessary, such as when mounting pipes by boring the wall of a nuclear power generating station or boring a thick working material such as the beam of a structure, conventionally the wet method must be relied on. This is because in the case of boring a deep hole by the dry method, even if the cooling fluid such as compressed air can be delivered along with the abrasive to and the distal end of the core bit, it is blocked in the narrow gap between the inner circumferential surface of the core bit and the inner circumferential wall of the cut groove that is being cut, and thus sufficient compressed air and abrasives cannot be supplied up to the distal end of the diamond segments. In particular, in the case of boring a deep hole by the dry method, there is the problem that the abrasives become obstructed before they arrive at the distal end of the core bit, and as a result, the diamond segments are not sufficiently dressed. Between diamond abrasive grains of the diamond segments or the regions that extend towards the upper axial direction from the diamond segments, the part indicated by the slanted line in FIG. 5 (shown in the figure by R), loading occurs due to the chips, the boring speed decreases due to the core bit becoming heated, and in the case of boring a hard working material such as hard reinforced concrete, there is the problem that boring can only be carried out part way. Therefore, when applying the dry method, in the case that the boring speed decreases, it is necessary to suspend the operation and extract and cool the core bit, or carry out the dressing of the diamond segments using a dressing, or replace the core bit, and these operations involve a considerable increase in labor and time.
For these reasons, conventionally boring has generally been carried out by the wet method. However, in most cases, work is carried out while people inhabit the structures that are the object of the work, and thus there are many times that it is desirable to avoid using this wet method that employs water because the sludge into which the chips and the dust are mixed that occurs due to the drying of the sludge causes deterioration of a residential environment. In contrast, in the case of the dry method using air or the like, the dust that is produced can be collected by a dust collecting apparatus.
Thus, in order to prevent contamination of the surrounding environment, there is a strong demand to carry out the boring of deep holes using the dry method, while at the same time there is the problem that when using the dry method, presently the labor and time for the boring operation increases drastically.
In consideration of the problems described above, it is an object of the present invention to provide an abrasive that can reduce the heat generation of the core bit due to boring and allow the boring of a deep hole while maintaining the boring speed when boring a deep hole using the dry method, and a boring method that uses the same.